Linear oxygen sensors are known, the so-called UEGO sensors (Universal Exhaust Gas Oxygen sensor), used, for example, in internal combustion engines for measuring the concentration of oxygen in the gases in a discharge and/or intake conduit, and thus obtaining information about the air/fuel (A/F) ratio at the discharge and/or intake.
These linear oxygen sensors are based on the use of electrolytic sensing cells, for example including zirconium dioxide (ZrO2), which are sensitive to the oxygen ions, and which generate suitable electrical signals depending on the quantity of oxygen present, when they come into contact with the gases.
In particular, linear oxygen sensors are known that use two electrolytic cells, generally defined as “pumping cell” and “sensing cell”, and linear oxygen sensors that envisage instead the use of a single electrolytic sensing cell.
For example, EP 1 001 261 A1, in the name of the present Applicant, discloses a control device, of an integrated microcontroller type, for a double cell linear oxygen probe.
Single-cell linear oxygen sensors may sometimes be preferable, for example for reducing the costs, size and circuit complexity of the associated control device.
As shown in the schematic sections in FIGS. 1a and 1b, a linear oxygen sensor, of a single-cell type, indicated in general by 1, comprises:
an electrolytic layer 2, including, for example zirconium dioxide, ZrO2;
a first and a second electrode 3a, 3b, in contact with the electrolyte layer 2, set on opposite sides in relation to the same electrolyte layer 2, and defining a first (positive pole) and, respectively, a second (negative pole) electrical terminal of the linear oxygen sensor 1;
a diffusion layer 4, above the first electrode 3a and in contact, during operation, with the gases whose oxygen concentration is desired to be measured, for example discharge gases (the second electrode 3b being placed in contact with an environment containing a reference air).
The linear oxygen sensor 1 furthermore comprises: a reference air duct 5, defining the aforesaid environment in contact with the second electrode 3b; and a heating element 6, set below the reference air duct 5, and suitably driven by applying an electrical quantity, to bring the electrolytic sensing cell to a suitable temperature (for example equal to 700°).
During operation, the cell current Ip that flows between the electrical terminals of the linear oxygen sensor 1 (denoted schematically with a current generator 7 in FIG. 1b), across which a biasing voltage ΔV of a suitable value is set (FIG. 1b shows a voltage generator ΔV in series with a resistor 8), is indicative of the oxygen percentage; this cell current Ip is consequently indicative of the air/fuel ratio A/F, as illustrated by way of example in FIG. 2.
It is known that, in the field of controlling linear oxygen sensors, the applied electrical quantities (in particular, the biasing voltage between the associated electrical terminals) are required to be within given (upper and lower) thresholds; exceeding (above or below) these thresholds may in fact cause the so-called phenomenon of “blackening” of the electrolyte, which is potentially damaging to the sensor or, in any case, sufficient to compromise its proper operation.
Furthermore, in the case of the previously illustrated single-cell linear oxygen sensor 1, the voltage applied between the corresponding electrical terminals is required to have a given relationship with the cell current Ip, with a suitable variation in correspondence with the variation of the same cell current Ip.